Giant rock crabs are massive versions of the weaker Rock crab. They lie in wait for prey, disguised as large boulders. When approached, they attack, unless they have become tolerant, in which case they remain as "boulders" - attempting to target them gives the message "Invalid target."

Giant Rock Crabs are good droppers of charms, as they drop 3 of them at a time. They also drop various black equipment (platebody and claws), worth a decent sum of coins. They are the only droppers of rock-shell parts, which are needed to create Rock-shell armour.

After the 27 May2009 update, Giant Rock Crabs dropped Charms in groups of three instead of ten. This hidden update initially angered players, though complaints died down as the rate of charm dropping was also increased.